Integrated analysis of steel and aluminium structures exposed to fire.

Through an intensive research activity an integrated tool for analysis of steel and aluminium structures exposed to fire has been developed. An integrated analysis of fire exposed structures includes three mains parts:

•  Fire analysis
•  Temperature analysis
•  Response analysis

Several models regarding each of these phenomena have been developed and introduced into the fire field. Such models could be both 2-dimensional and 3-dimensional. However, the link between the different programs and thus the combined usability has not been satisfactory. Therefore the idea with respect to development of an integrated tool for analysis of the total event, from the fire starts to the final structural response, is supported by the users on all levels.

The integrated program developed through the above mentioned research activity gives the possibility of performing a complete analysis of complex structures exposed to different fire scenarios. The program consists of the three parts: FIREINT - which is a transient 3D finite volume computer code for solving the conservation equations for mass-balance, momentum and energy to analyse the fire development and thermal load; FAHTS - which is a transient temperature analysis program representing the interface between the fire simulation and the mechanical response analysis; USFOS - which has been especially developed for assessment of offshore structures subjected to a variety of accidental loads, here to calculate progressive collapse of large scale structures for a given temperature field history.

A large scale test on a 3 dimensional steel structure has been conducted at SINTEF NBL as (Norwegian Fire Research Laboratory). The objective of the test was to compare the calculation results from the integrated analysis tool with results from a real fire test, that means a valuable verification. The structure should be quite representative of structural systems used offshore and the fire load should be meaningful compared to actual fire scenarios.

A test structure comparable to the upper part of a four legged jacket was exposed to a simulated gas pool fire ( 1 m x 1 m) as an idealized representation of a liquid pool fire. The dimensions of the structure were 3,9 m x 3,9 m x 3,9 m. At the top the structure was fitted with a lattice girder which is a somewhat simplified model of a typical deck structure.

A constant force of P= 300 kN was applied vertically at the fire exposed frame corner prior to ignition of the gas burner. The temperature distribution in the structure and the flame was measured by approximately 150 thermocouples. A thermocamera was also used in the test to compare within the temperature field calculated by FAHTS. The total heat flux was measured at selected locations on and near the structure. Strains gauges and load cells were used to assess the mechanical load history in the structure.

During the fire test the heated leg and adjacent diagonals got a rapid temperature rise. Thermal expansion and vertical movement initiated a partial collapse due to buckling. The load was transferred to other parts of the 3 dimensional structure and a progressive collapse occurred, - as estimated. This was a controlled collapse due to the fact that the fire behaviour of the complete structure had been calculated in advance by using the integrated tool. The estimated performance was also shown by an animation video prior to the test, - and as the tested structure responded just as estimated, it happened to be a very successful verification.

Through the development, integration and verification of computer programs for fire development, thermal exposure, temperature distribution and structural response an efficient integrated computational tool for analysis of the fire behaviour of structures has been established. This gives the possibility of analysing more complete structures exposed from different fire scenarios, and therefore it will hopefully contribute to bring the fire engineering field a significant step forward.

Acknowledgement:
The above mentioned research activity has been sponsored by Royal Norwegian Council for Scientific and Industrial Research as well as the five companies Elf Petroleum Norge A/S, Hydro Aluminium Structures a.s., Phillips Petroleum Company Norway, Saga Petroleum a.s. and Statoil a.s.

Contact person:
Mette Kristin Ulfsnes
Fax: (+47) 73 59 10 44
Telephone: (+47) 73 59 10 78
E-mail: